Cellular mediators of renal vascular dysfunction in hypertension

Abstract

The renal vasculature plays a major role in the regulation of renal blood flow and the ability of the kidney to control the plasma volume and blood pressure. Renal vascular dysfunction is associated with renal vasoconstriction, decreased renal blood flow, and consequent increase in plasma volume and has been demonstrated in several forms of hypertension (HTN), including genetic and salt-sensitive HTN. Several predisposing factors and cellular mediators have been implicated, but the relationship between their actions on the renal vasculature and the consequent effects on renal tubular function in the setting of HTN is not clearly defined. Gene mutations/defects in an ion channel, a membrane ion transporter, and/or a regulatory enzyme in the nephron and renal vasculature may be a primary cause of renal vascular dysfunction. Environmental risk factors, such as high dietary salt intake, vascular inflammation, and oxidative stress further promote renal vascular dysfunction. Renal endothelial cell dysfunction is manifested as a decrease in the release of vasodilatory mediators, such as nitric oxide, prostacyclin, and hyperpolarizing factors, and/or an increase in vasoconstrictive mediators, such as endothelin, angiotensin II, and thromboxane A₂. Also, an increase in the amount/activity of intracellular Ca²⁺ concentration, protein kinase C, Rho kinase, and mitogen-activated protein kinase in vascular smooth muscle promotes renal vasoconstriction. Matrix metalloproteinases and their inhibitors could also modify the composition of the extracellular matrix and lead to renal vascular remodeling. Synergistic interactions between the genetic and environmental risk factors on the cellular mediators of renal vascular dysfunction cause persistent renal vasoconstriction, increased renal vascular resistance, and decreased renal blood flow, and, consequently, lead to a disturbance in the renal control mechanisms of water and electrolyte balance, increased plasma volume, and HTN. Targeting the underlying genetic defects, environmental risk factors, and the aberrant renal vascular mediators involved should provide complementary strategies in the management of HTN.

the role of the kidney in the regulation of sodium and water balance and blood pressure (BP) is well recognized (53, 55, 128). A decrease in plasma volume and renal blood flow (RBF) stimulates renin release from the juxtaglomerular cells (JGCs) and activates the renin-angiotensin system (RAS). Renin transforms angiotensinogen to angiotensin I (ANG I), and angiotensin-converting enzyme (ACE) transforms ANG I to ANG II. ANG II stimulates the adrenal cortex to release aldosterone (Aldo), which acts on the renal collecting ducts, stimulates sodium and water absorption, and thereby restores plasma volume and RBF. The increased ANG II and intrarenal pressure then inhibit further renin release from JGCs (53). Other components of RAS, such as ANG III and IV (products of ANG II conversion by aminopeptidases), ANG 1-7 (product of ANG II conversion by ACE2), and (pro)renin receptor (in the kidney and other tissues), are potential participants in sodium balance and BP regulation.

In addition to renal control of water and electrolytes, regulation of renal vascular resistance (RVR) is critical to maintain RBF and kidney function. This involves rigorous control of blood flow through an intricate network of renal segmental, interlobar, arcuate, and interlobular arteries, down to afferent and efferent arterioles in the renal cortex, and vasa recta in the medulla. The kidney receives ∼20% of cardiac output. In the glomeruli, 20% of total renal plasma flow filters into Bowman's space, 70% goes through the efferent arterioles to the peritubular capillaries in the renal cortex, and 5–10% go to the medulla. To maintain salt and water balance, there is a need to control RBF and glomerular filtration rate (GFR). This is achieved via an array of local, paracrine, neural, endocrine, and systemic control mechanisms. For instance, renal autoregulation uses local and paracrine mechanisms to effectively control RBF. An increase in renal perfusion pressure activates renal autoregulation, leading to renal vasoconstriction and a reduction of RBF to a maintained normal level (103, 131). Renal autoregulation is partly achieved via a myogenic mechanism within the vascular smooth muscle (VSM) of preglomerular vessels. Another mechanism of renal autoregulation is tubuloglomerular feedback (TGF), in which the macula densa, a plaque of specialized tubular epithelial cells, senses the increases in filtered and tubular NaCl and sends a signal(s) to induce afferent arteriolar vasoconstriction, leading to a decrease in RBF and GFR, and thereby reduction of filtered NaCl. Thus localized release of vasodilators and vasoconstrictors in the kidney could act in a paracrine fashion to affect renal vascular tone (119). Renal vascular tone and TGF may also be modulated by additional neural, endocrine, and systemic agents. Circulating ANG II affects the cardiovascular centers in the brain to enhance sympathetic nerve firing to systemic and renal blood vessels. While sympathetic innervation of systemic vessels is largely involved in moment-to-moment restoration of BP in case of a sudden decrease, sympathetic innervation of renal vessels may affect renal function and long-term regulation of BP (100, 106). Also, circulating ANG II directly constricts blood vessels, leading to systemic and renal vasoconstriction and maintenance of BP.

Thus BP regulation involves complex interaction between neural, hormonal, vascular, and renal control mechanisms. To maintain RBF and kidney function, the renal vessels are constantly adjusting their diameter and tone. In addition to renal autoregulation, short-term adjustment of renal vascular tone involves changes in endothelial cell function, neurohormonal drive, and VSM sensitivity to vasoconstrictor stimuli. Long-term adjustment involves vascular wall hypertrophy and structural changes in the vessel wall architecture and extracellular matrix (ECM). Excessive changes in endothelial cell function (decreased vasodilators, increased vasoconstrictors), VSM function (increased VSM tone and reactivity), and ECM composition (remodeling) could lead to renal vascular dysfunction and vasoconstriction (Fig. 1). Persistent renal vasoconstriction causes increases in RVR and could increase BP, in part, by contributing to total vascular resistance. Also, renal vasoconstriction in preglomerular arterioles decreases RBF and filtered NaCl, leading to activation of TGF and increased renin and ANG II release. The effects of ANG II and other vasoconstrictors are normally counterbalanced by vasodilators, such as nitric oxide (NO) and prostacyclin (PGI₂). However, when the release of vasodilator mediators is compromised, ANG II acts unopposed in a paracrine and systemic fashion to induce further renal vasoconstriction. The released ANG II could also increase Aldo secretion and stimulate NaCl and water absorption. Additionally, ANG II-induced vasoconstriction of postglomerular efferent arterioles in the renal cortex, and vasa recta in the renal medulla, leads to further increases in sodium absorption and increased plasma volume and BP (Fig. 1). Long term, the increased BP causes renal vascular remodeling and further increases in renal vasoconstriction and RVR, leading to a vicious cycle, persistent increases in BP and hypertension (HTN).

Fig. 1.

Mediators of renal vasoconstriction and hypertension (HTN). Genetic mutation of an ion channel or transporter, combined with dietary factors, oxidative stress, inflammatory response, and mental stress, cause vascular, renal, and neural dysfunction. Endothelial dysfunction causes a decrease in vasodilators (VDs) [nitric oxide (NO), prostacyclin (PGI₂), endothelium-derived hyperpolarizing factor (EDHF)] and an increase in vasoconstrictors (VCs) [angiotensin II (ANG II), endothelin-1 (ET-1), thromboxane A₂ (TxA₂)]. Vascular smooth muscle (VSM) dysfunction is associated with increased Ca²⁺, PKC, Rho kinase, and MAPK and leads to VSM contraction and growth. Sympathetic nerves contribute to VSM hyperactivity. Changes in the expression/activity of matrix metalloproteinases (MMPs) and tissue inhibitors of MMPs (TIMPs) in extracellular matrix (ECM) promote renal vascular remodeling. Excessive renal vasoconstriction increases renal vascular resistance (RVR) and contributes to total vascular resistance. Also, vasoconstriction of preglomerular arterioles decreases renal blood flow (RBF) and filtered NaCl, leading to activation of tubuloglomerular feedback (TGF) and increased release of renin and ANG II, which acts in a paracrine or systemic fashion to induce further vasoconstriction. ANG II also increases aldosterone (Aldo) secretion and stimulates tubular NaCl and water absorption. ANG II-induced vasoconstriction of postglomerular efferent arterioles in the cortex and vasa recta in the medulla further increases NaCl absorption, plasma volume, and blood pressure (BP). HTN, in turn, promotes vascular and renal inflammation and oxidative stress, leading to a vicious cycle and progression of HTN. Not shown, vascular agents could act in a paracrine fashion and affect tubular reabsorption, sympathetic nerves may affect renin release from juxtaglomerular cells (JGCs), and renal afferent nerves may send feedback signals to sympathetic centers. ENaC, epithelial Na channel; ROS, reactive oxygen species; EET, epoxyeicosatrienoic acid; HETE, hydroxyeicosatetraenoic acid; NE, norepinephrine; GFR, glomerular filtration rate.

Several reviews have provided detailed information on systemic vascular tone and renal tubular function as important control mechanisms of BP (16, 53, 55, 128, 138). However, the renal vessels are unique in that they are both anatomically and physiologically linked to renal glomerular filtration and tubular function. To highlight the close relationship between the vascular and renal mechanisms, this review will take a more integrative approach to discuss the potential predisposing factors and cellular mediators involved in renal vascular dysfunction and how they could affect the renal glomerular filtration and tubular function and lead to HTN. Because the amount of information could be enormous when describing all forms of HTN, and may be limited if discussing only one form of HTN, we will focus on two common forms, namely genetic and salt-sensitive (SS) HTN. The review will describe evidence of renal vascular dysfunction as a primary cause or consequence of HTN. The genetic predisposition, environmental and dietary risk factors, and the role of vascular inflammation and oxidative stress in HTN will be briefly described. The changes in renal hemodynamics, the vasodilatory and vasoconstrictive mediators involved, and the abnormalities in endothelium, VSM, and ECM will then be discussed. Throughout the review, we will discuss hemodynamic data in human then animal models, physiological evidence in large then small vessels, regional differences in the renal cortex then the medulla, followed by discussion of the cellular and molecular mechanisms involved. Future directions to further delineate the cellular mediators of renal vascular dysfunction and the clinical implications in treatment of HTN will also be discussed.

Renal Vascular Dysfunction as a Cause or Consequence of HTN

Although renal vascular dysfunction, increased RVR, and changes in kidney function are demonstrated in several forms of HTN, whether their relationship is causative or associative in nature has not been clearly established. The preglomerular vasculature are resistance vessels and therefore are likely to demonstrate the same structural and functional alterations that occur in other peripheral resistance vessels as a consequence of HTN. While this may be the case in some essential hypertensive patients, assessment of the renal hemodynamics in prehypertensive humans suggests that an increase in RVR may constitute an initial event that leads to elevation of total vascular resistance and BP (see Fig. 1). Studies have shown an increase in RVR but normal RBF and GFR in young people with a family history of HTN compared with control subjects, suggesting that an abnormal renal vasoconstriction exists in the prehypertensive state (135). Also, offspring of hypertensive parents demonstrate an exaggerated renal vasodilation in response to Ca²⁺ channel blockers, supporting the existence of functional renal vasoconstriction in subjects at risk of developing HTN (135). Borderline HTN is a condition in which a subject's BP is above normal, but not sufficiently high to require antihypertensive therapy, and may represent an early phase of essential HTN. Although RBF tends to be normal or slightly reduced in borderline HTN patients, RVR is elevated. Borderline hypertensive subjects also exhibit an enhanced renal vasoconstrictor response to infusion of norepinephrine. Thus, while increased preglomerular vascular tone may be due to a generalized disturbance in peripheral vascular resistance, it could also be due to localized alterations in the renal vasculature and could lead to HTN (135).

The primary importance of renal vascular alterations in the development of HTN is further supported by experimental studies. Experimental cross-transplantation studies suggest that the underlying pathology of HTN travels with the kidney from spontaneously hypertensive rat (SHR) donors to normotensive recipients, and kidneys from normotensive donors lower BP in transplanted SHR (27, 51). Measurement of the renal hemodynamics in SHR showed increased RVR as HTN develops, while GFR and glomerular capillary pressure are normal, and the unaltered glomerular function may be due, in part, to a renal autoregulatory adjustment to the increase in BP. However, in young SHR, an increase in RVR and renal vascular hypertrophy occurs as early as 4 wk of age, during which there is no established HTN (159). Also, in prehypertensive young SHR, there is increased reactivity of the renal vasculature to vasoconstrictors and decreased medullary blood flow, supporting a role in the development of HTN (139, 180). Furthermore, in SHR, preglomerular arterial wall hypertrophy is not reversed by antihypertensive treatment, which is not the case in other vascular beds (158). The presence of structural and functional changes in preglomerular and afferent arterioles before the development of HTN and the persistence of structural changes, despite normalization of BP, suggest that these renal vascular changes are not a mere consequence of elevated BP, but may be involved in the pathogenesis of HTN.

Genetic Predisposing Factors in Renal Vascular Dysfunction

Essential HTN, a form of HTN with no clear cause, is believed to have a hereditary influence. Also, BP is similar in descendants of the same family, but is different between adopted and biological siblings, supporting a genetic influence on BP. Linkage studies utilizing whole genome scans have identified chromosomal regions with potential relation to BP regulation. Studying sibling pairs with discordant BP (one sibling with high BP and one with low BP) as well as concordant pairs (both with high BP or both with low BP) led to the identification of several regions on chromosomes 3, 11, 15, 16, and 17 that could be linked to BP regulation. Further studies suggested that chromosome 15 is the most likely region to harbor a HTN gene (189).

Genetic studies have identified 20 genes associated with changes in BP in humans. Also, gene-targeting experiments in mice have identified over 30 genes, for which inactivating or activating mutations trigger chronic changes in BP. Most of these gene mutations involve an ion channel, an ion transporter, or a regulatory enzyme that could affect the renal function (84, 110) (Fig. 2). Some of these channels, transporters, and enzymes are expressed in various blood vessels (9, 44, 73, 183, 194), and mutations in their genes in the renal vasculature could significantly affect renal vascular function. Examples of channels, transporters, and enzymes involved in BP regulation include epithelial Na channel (ENaC), RAS, Na⁺/H⁺ exchanger (NHE), and Na⁺/Ca²⁺ exchanger (NCX).

Fig. 2.

Genetic factors in renal and vascular dysfunction and HTN. Genetic mutation in the Na⁺/H⁺ exchanger (NHE), Na⁺-Cl⁻ cotransporter (NCC), ENaC, Na⁺-K⁺-2Cl⁻ cotransporter (NKCC2), and Na⁺-K⁺ pump in the nephron could lead to sodium and water retention. Changes in Aldo synthesis and mineralocorticoid receptor (MR), as well as ANG II and ANG II type 1 receptor (AT₁R), contribute to sodium absorption. Mutations in the genes expressing ENaC, NHE, and Na⁺/K⁺/2Cl⁻ and Na⁺/Ca²⁺ exchanger (NCX) in VSM could promote renal vasoconstriction and exacerbate HTN.

ENaC.

The amiloride-sensitive ENaC is responsible for sodium absorption in the cortical collecting tubules. Mutations in the genes encoding β- and γ-subunits of ENaC result in increased channel activity and sodium absorption. Given the impact of ENaC activity on H⁺ and K⁺ secretory mechanisms in the principal cells of the cortical collecting tubules, mutations in ENaC result in hypokalemic alkalosis, low renin SS-HTN, and low-plasma Aldo, all characteristics of Liddle's syndrome (110). Studies suggest the presence of β- and γ-ENaC in mouse renal interlobar VSM, where they may play a role in the renal vascular mechanosensitivity and the myogenic response to intravascular pressure (73). The regulation of renal vascular ENaC during high dietary sodium intake and in hypertensive compared with normotensive individuals is an important area for investigation.

RAS.

RAS plays a role in the regulation of sodium balance and BP, and ANG II acts via ANG II type 1 receptor (AT₁R) to induce renal vasoconstriction, Aldo secretion, and sodium reabsorption. Molecular variants of individual components of RAS may contribute to genetic predisposition to HTN. Linkage studies have suggested a relation between the chromosomal region harboring the AT₁R gene and HTN (80). At least 25 polymorphisms in AT₁R gene have been described, and the A1166C polymorphism (adenine/cytosine substitution at position 1166) is associated with severe HTN (10). A link between functional gene copies of AT₁R and BP is also observed in mice, and the expression of AT₁R gene in the aorta, brain, and kidney is regulated by salt intake (110). Studies have suggested a role of AT₂R gene (+1675 G/A) on left ventricular structure in humans (145). Also, the higher BP in male compared with female SHR has been related to decreased expression of ANG II type 2 receptor (AT₂R) in the aorta and mesenteric microvessels of males (155). Whether changes in the relative expression of AT₁R and AT₂R also occur in the renal vasculature of SHR remains to be examined. With regard to ACE, linkage studies between I/D polymorphism of ACE gene (insertion/deletion of a 287 bp in intron 16) and HTN gave mixed results, but an association with salt sensitivity has been suggested (1, 49, 144). The relation between ACE and BP was also tested in mice having one, two, or three functional copies of ACE gene at its chromosomal location. Although serum ACE activity increased progressively from the one-copy to three-copy mice, BP did not differ, leading to the suggestion that I/D ACE polymorphism may affect BP only during salt loading (91). Studies in mice strains with functional copies of the angiotensinogen gene suggest an association between plasma angiotensinogen levels and BP. Also, the M235T polymorphism (methionine/threonine substitution at codon 235) in angiotensinogen gene has been associated with HTN; however, its association with salt sensitivity yielded mixed results (61, 74, 148). Additional information on the expression of ANG II receptors and other components of RAS in the kidney and renal vasculature and their role in HTN are described below under mediators of renal endothelial dysfunction.

NHE, Na⁺-Cl⁻ and Na⁺-K⁺-2Cl⁻ cotransporter.

The proximal tubules account for 60% of sodium reabsorption. NHE3 mediates a considerable fraction of proximal tubular sodium uptake and may play a role in HTN. NHE activity is increased in erythrocytes and lymphocytes of patients with essential HTN (110). Also, apical membrane vesicles from kidney of young prehypertensive SHR show increased sodium uptake via NHE (2). SS Dahl/Rapp rats have reduced ability to decrease their renal NHE3 exchange rate in response to high-salt diet compared with salt-resistant rats (95). Also, transgenic mice overexpressing the NHE in the renal tubules become hypertensive during salt loading (13). Gene mutations in the Na⁺-Cl⁻ cotransporter in the distal tubules and Na⁺-K⁺-2Cl⁻ cotransporter in the Loop of Henle could also affect sodium and water absorption and BP (Fig. 2). The NHE and Na⁺-K⁺-2Cl⁻ have been identified in vascular tissues and could affect the intracellular pH and renal vascular function. This is supported by the report that inhibition of the Na⁺-K⁺-2Cl⁻ cotransporter affects the myogenic and ANG II responses in the rat afferent arteriole (183).

NCX.

In VSM, NCX promotes Ca²⁺ extrusion and lowers intracellular Ca²⁺ concentration ([Ca²⁺]_i) (9). NCX activity is greater in afferent than efferent arterioles (8). Protein kinase C (PKC) upregulates NCX in afferent and efferent arterioles and mesangial cells in Dahl salt-resistant, but not Dahl SS, rats (8). In SHR, Dahl SS, and Dahl/Rapp SS rats, NCX activity is suppressed mainly in afferent arterioles, leading to accumulation of intracellular Ca²⁺ and vasoconstriction (8).

The identification of these gene mutations supports the concept of genetic predisposition to HTN. Also, most single-gene disorders with an effect on BP involve a defect in renal sodium reabsorption, emphasizing the role of the kidney. The identification of some of these channels and transporters in the renal vasculature raises the possibility that their genetic mutation could affect renal vascular function. However, studies of the human genome did not find an association between a particular locus or single-gene defect and the large variation in BP seen in HTN, suggesting multiple loci, each of which can contribute to HTN.

Role of Dietary Salt Intake, Vascular Inflammation, and Oxidative Stress

In genetically predisposed individuals, other factors, such as dietary salt intake, vascular inflammation, and oxidative stress, may promote renal vascular dysfunction and HTN (Fig. 1).

Dietary salt intake.

SS-HTN is defined as an increase in BP to an increase in dietary salt intake. In humans with SS-HTN and in rat models of SS-HTN, a high-salt diet is associated with renal and vascular inflammation, oxidative stress, increased reactive oxygen species (ROS), and endothelial cell dysfunction that eventually lead to renal vasoconstriction and increased BP (Fig. 1). SS-HTN involves a genetic defect, combined with subtle acquired renal injury. Genetic abnormalities in the renal transport mechanisms in proximal tubules and Loop of Henle cause increased NaCl delivery to the macula densa, leading to suppression of renin release, and thereby impair the kidney's ability to suppress renin release during high sodium intake and cause SS-HTN (55). Renal abnormalities that cause loss of functional kidney mass and inability to modulate RAS could also lead to SS-HTN. For example, renal vascular dysfunction, arteriolopathy, and tubulo-interstitial injury could lead to renal hypoxia-ischemia, inflammation, and loss of functional nephrons (78). With loss of functional nephrons, surviving nephrons attempt to maintain balance and excrete greater amounts of sodium and water by increasing GFR and decreasing tubular sodium reabsorption, resulting in increased NaCl concentration in distal tubules and macula densa, and leading to suppression of renin release and impaired kidney's ability to decrease renin secretion during high sodium intake. Long term, the loss of functional nephrons and persistent compensatory vascular and renal changes eventually damage surviving nephrons, leading to glomerulosclerosis, decreased glomerular capillary filtration coefficient, and SS-HTN (12).

Vascular inflammation.

There is a potential link between vascular inflammation and HTN. Cross-sectional studies in hypertensive individuals have shown increased plasma and vascular tissue levels of C-reactive protein, cytokines such as TNF-α and IL-6, chemokines such as monocyte chemoattractant protein-1 and plasminogen activator inhibitor-1, and adhesion molecules such as P-selectin and soluble ICAM-1 (20). However, whether inflammation causes structural or functional changes in the renal vasculature and lead to HTN, or is just a consequence of HTN, is unclear. Studies with ANG II have supported a relationship between vascular inflammation, renal vascular dysfunction, and HTN. ANG II stimulates TNF-α production by human monocytes and the adherence of monocytes to endothelium (54). Also, ANG II infusion for 3 days in rats is associated with increased TNF-α production by renal glomerular endothelial cells and tubular and vascular cells (136). ANG II also stimulates renal and vascular IL-6 production and activates transcription factor NF-κB which further increases the production of cytokines, chemokines, and adhesion molecules (136). Interestingly, the HTN associated with ANG II infusion is attenuated in IL-6 knockout mice (94). The role of IL-6 in HTN partly involves increased AT₁R density in proximal tubules, leading to increased sodium and water reabsorption and ANG II-dependent HTN (94). IL-6 may also upregulate AT₁R and enhance the effects of ANG II on vascular inflammation, oxidative stress, and vasoconstriction (184). ANG II increases ROS, which further promotes the inflammatory process. The role of ROS in ANG II responses is supported by the observation that ANG II-induced cytokine production is inhibited by free-radical scavengers (90). In HTN, the mechanical stress on arterial wall and the release of proinflammatory humoral stimuli, such as ANG II and ET-1, are thought to induce oxidative stress, which could cause vascular inflammation (50, 172). The inflammatory factors stimulate ECM deposition and promote hypertrophy/hyperplasia of VSM cells (174). Inflammation, in turn, increases oxidative stress, leading to vicious cycle, chronic vascular inflammation, and atherosclerosis. Collectively, in HTN, inflammation is associated with endothelial dysfunction and vascular remodeling, either directly or indirectly through oxidative stress (24).

Oxidative stress.

Oxidative stress is an important factor in the regulation of renal vascular function and BP. Oxidative stress is an imbalance between ROS production and the ability of biological system to scavenge or metabolize ROS. Most animal models of HTN have oxidative stress, and antioxidants decrease BP in hypertensive animals. Among ROS, superoxide (O₂^•−) has high reactivity and ability to scavenge NO. Also, H₂O₂ reacts with metals to form highly reactive hydroxyl radical (OH⁻). OH⁻ and O₂^•− react with NO to form peroxynitrite (ONOO⁻). O₂^•− oxidizes the endothelial NO synthase (eNOS) cofactor tetrahydrobiopterin to dihydrobiopterin, causing uncoupling of eNOS and more O₂^•− production. NADPH oxidase produces O₂^•− in the renal cortex. In the medulla, NADH oxidase, NADPH oxidase, and mitochondrial respiratory enzymes produce O₂^•− (196). NADPH oxidase is also widely expressed in the renal vasculature.

ROS can cause renal vasoconstriction by decreasing NO bioavailability (146). O₂^•− may also mediate the acute effects of ANG II, endothelin (ET)-1, and catecholamines on renal vasoconstriction by a mechanism independent of NO (79). ROS also exert vasoconstriction in the renal microcirculation indirectly by stimulating the release of F2 isoprostane and adenosine (23). In some vascular beds, ROS may promote vasoconstriction directly by increasing VSM [Ca²⁺]_i (101), or indirectly by inhibiting the production of other vasodilators, such as PGI₂ (197), and the role of these mechanisms in the renal microcirculation remains to be defined.

In the renal cortex, basal O₂^•− production plays a role in TGF. Increased NaCl delivery and flow-associated mechanical factors induce NADPH oxidase in macula densa to produce O₂^•−. O₂^•− enhances TGF directly by constricting afferent arterioles, indirectly by scavenging NO in the macula densa, which tonically inhibits TGF (98), and by increasing the production and vasoconstrictor action of adenosine (23) (Fig. 3). Under basal conditions, the renal medulla produces more O₂^•− than the cortex, owing to its lower Po₂, and O₂^•− may participate in the regulation of renal medullary blood flow (196). Oxidative stress and O₂^•− levels are also affected by vascular mediators, such as ANG II and ET. ANG II induces oxidative stress and upregulation of NADPH oxidase and increases ROS formation in mitochondria, and some of these effects are mediated by Aldo (48, 137). ANG II-induced renal interstitial influx of phagocytes and cytokines also increases O₂^•− production. O₂^•−, in turn stimulates NF-κB, which activates cytokine-encoding genes and proinflammatory response in the kidney, leading to a vicious cycle. Thus ROS modulate the renal hemodynamics, both directly, by causing vasoconstriction and sodium reabsorption and indirectly, by inducing renal inflammation.

Fig. 3.

TGF and autoregulation of RBF and renin release. A decrease in RBF is associated with increased neuronal NO synthase (nNOS) activity, NO-mediated renin release by JGC, and formation of ANG II. ANG II inhibits further renin release. Paracrine signaling by macula densa also controls RVR and renin release. Increased luminal NaCl alters macula densa cell Na⁺ concentration, pH, volume, basolateral membrane potential, and intracellular Ca²⁺ concentration ([Ca²⁺]_i). Luminal ANG II via AT₁R and consequent changes in Na/H/X2 exchanger and Cl/HCO₃⁻/X at the apical membrane and Na/H/X4 at the basolateral membrane affect macula densa Na⁺ transport, cell alkalinization, and volume. NaCl entry via apical NKCC2 and Cl exit through basolateral channel lead to cell depolarization and increased [Ca²⁺]_i. Moderate increases in luminal NaCl prompt macula densa cells to release the VCs TxA₂ and ATP and to decrease vasodilation by prostaglandin E₂ (PGE₂). ATP diffuses through ATP-permeable maxi-anion channels and stimulates P₂ receptors and vasoconstriction. ATP is also dissociated to adenosine, which binds to A₁ and A₄ receptors, causing further afferent arteriolar constriction and decreased renin release from JGCs. Macula densa intracellular Na⁺ is also regulated by apical Na⁺-K⁺-ATPase. Normally, large increases in luminal NaCl stimulate the release of NO, which prevents excessive TGF-mediated vasoconstriction. An increase in oxidative stress and superoxide (O₂^•−) decreases NO bioactivity and antagonizes its inhibitory effect on TGF. ACE, angiotensin-converting enzyme.

Urinary excretion of oxidative stress biomarkers, such as 8-iso-PGF_2α and thiobarbituric acid reactive substances, is increased in HTN (151). During the development of HTN in SHR, increased ROS inactivates NO and leads to diminished buffering by NO in JGC and enhanced TGF and preglomerular vasoconstriction (33, 185). An increase in AT₁R-mediated formation of O₂^•− appears to diminish NO signaling in JGCs of SHR (185). In the renal medulla of SHR, ANG II-induced vasoconstriction is reversed by intramedullary infusion of the NO synthase (NOS) substrate l-arginine, suggesting that there is less NO to buffer the actions of ANG II, possibly due to increased O₂^•− and NO scavenging (35). Also, in SHR, treatment with the O₂^•− dismutase mimetic tempol decreases RVR and BP, largely by increasing NO bioavailability in the renal vasculature and JGCs (146).

O₂^•− also plays a role in SS-HTN. In Dahl-SS rats, high-salt diet is associated with rapid decline in medullary blood flow and increased BP, which can be prevented by infusing l-arginine in the renal medulla, suggesting reduced capacity to generate NO (114). In Dahl SS-HTN, there is decreased GFR and RBF, increased O₂^•− production, BP, renal tubulointerstitial damage, and glomerular necrosis, which are reversed by antioxidants (172). Also, O₂^•− is elevated in sympathetic neurons of DOCA-salt hypertensive rats via activation of NADPH oxidase (28). Increased salt intake is also associated with enhanced renal expression/activity of NADPH oxidase, generation of O₂^•−, and diminished expression of SOD (86).

The increased oxidative stress, combined with the aforementioned genetic predisposition, dietary salt intake, and vascular inflammation adversely affect the renal vascular endothelial cell function, VSM, and ECM and lead to significant elevation of RVR, BP, and HTN (Fig. 1).

Renal Endothelial Cell Dysfunction in HTN

There is a debate whether endothelial cell dysfunction is a cause or a consequence of HTN. The debate toward the consequence side stems from the observations that endothelial dysfunction is found in all forms of HTN, increases with increases in BP, and is partly reversed by antihypertensive therapy. Other studies have demonstrated that adult SHR have both endothelial dysfunction and overt HTN, while young SHR have endothelial dysfunction but normal BP, suggesting that endothelial dysfunction may be a cause rather than a consequence of HTN (124). Endothelial cell dysfunction involves an imbalance in the release of vasodilatory mediators, such as NO, PGI₂, and endothelium-derived hyperpolarizing factor (EDHF), and vasoconstrictive mediators, such as ET, ANG II, and thromboxane A₂ (TxA₂) (Fig. 4).

Fig. 4.

Endothelium-derived renal VDs and VCs in HTN. In renal vascular endothelial cells, Ca²⁺ release from the endoplasmic reticulum (ER) increases endothelial NO synthase (eNOS) activity and NO production. NO diffuses into renal VSM cells, activates guanylate cyclase (GC), and increases cGMP. cGMP causes VSM relaxation by inhibiting Ca²⁺ influx and stimulating Ca²⁺ extrusion mechanisms. Activation of cyclooxygenases (COX) increases PGI₂ production, which activates adenylate cyclase (AC) and increases cAMP in VSM. cAMP causes VSM relaxation by mechanisms similar to those of cGMP. Renal endothelial release of EDHF, such as certain EETs, activates K⁺ channels and causes hyperpolarization of VSM and inhibition of Ca²⁺ influx through Ca²⁺ channels. The endothelium also releases ET-1, ANG II, and TxA₂, which act on specific receptors in VSM to cause renal vasoconstriction. Activation of endothelial ET_B1R and AT₂R is coupled to increased release of VDs and renal VSM relaxation. A decrease in endothelium-derived VDs and an increase in endothelium-derived VCs are associated with renal vasoconstriction, increased RVR, and HTN. IP₃, inositol trisphosphate; PIP₂, phosphatidylinositol 4,5-bisphosphate; DAG, diacyglycerol; TP, thromboxane-prostanoid; SR, sarcoplasmic reticulum.

NO.

NO is a major vasodilator and regulator of the renal hemodynamics and BP (7). NO is produced by NOS using l-arginine as substrate. Three NOS isoforms, NOS1 [neuronal NOS (nNOS)], NOS2 [inducible NOS (iNOS)], and NOS3 (eNOS), are widely distributed in the kidney (7). Constitutive NOS3 (eNOS) in microvascular endothelium and NOS1 (nNOS) in the macula densa produce a large portion of the basal NO release required to maintain RBF and GFR (185). However, the cellular source of NO may not be restricted to the endothelium and macula densa. As NOS isoforms are constitutively expressed by many cell types of the nephron, it is possible that NO produced by neighboring epithelial cells, such as proximal tubules or renal connecting tubules, could diffuse to microvascular smooth muscle cells and, consequently, influence vascular tone. NOS inhibition in rats by intravenous infusion of N^ω-nitro-l-arginine methyl ester (l-NAME) at a dose that does not affect systemic BP, increases RVR and decreases RBF, GFR, diuresis, and natriuresis, suggesting that NO exerts a greater tonic vasodilator effect in the renal vasculature than in other systemic vascular beds (93, 154). NO controls sodium excretion via tubular action, regulation of medullary vascular tone, and pressure-natriuresis. NO also causes vasodilation of afferent and efferent arterioles (32); blunting of TGF-mediated constriction of afferent arterioles in response to high NaCl delivery at macula densa (188); counterbalance of renal vasoconstriction caused by ANG II, ET-1, and TxA₂ (Fig. 5); and inhibition of sodium absorption in proximal and distal nephron (29, 163).

Fig. 5.

Regulation of RBF by VCs and VDs. Decreased RBF is associated with increased renin and ANG II release. The vasoconstrictive effects of ANG II and other VCs are normally counterbalanced by VDs, such as NO and PGI₂, leading to maintained RBF and GFR. When the release of VD mediators is compromised, ANG II acts unopposed to induce renal vasoconstriction, leading to decreased RBF and GFR, which, in turn, promote renin release, further activate renin-angiotensin system, and lead to progression of HTN.

Impaired endothelial NO production is observed in offsprings of essential hypertensive patients (165). Also, normally, high dietary salt intake is associated with increased NO production to increase salt excretion and maintain normal BP (153). This may explain the observation that, in normotensive rats, high salt diet blunts TGF, and NOS blockade is associated with enhanced TGF. This response to NOS blockade during HS intake is lost in SS-Dahl/Rapp rats, supporting reduced NO production in SS-HTN (188). The renal myogenic autoregulatory mechanism is also impaired in Dahl-SS rats during high-salt diet (81). Also, afferent arteriolar vasodilatation by NO is impaired in Dahl-SS rats due to increased free radical (123). Studies in dogs have also shown that NOS inhibition by l-NAME increases O₂^•− in the renal vasculature, indicating a protective role of NO against the effects of O₂^•− (105). Endothelial cell dysfunction may also involve increased arginase activity, which converts l-arginine to l-ornithine and urea, thus decreasing the substrate for NO production (76). H₂O₂ upregulates arginase activity and thereby may impair renal vasodilatation (170).

PGI₂.

Eicosanoids may play a role in the regulation of renal vascular function and BP. Eicosanoids are synthesized from arachidonic acid in a compartmentalized fashion, such that afferent arteriolar endothelial cells produce mainly PGI₂ by cyclooxgenase (COX) (161), glomeruli mainly PGE₂ and TxA₂, and interstitial cells and collecting ducts mainly PGE₂ (115). PGI₂ diffuses through the intima, increases cAMP in VSM, and promotes vascular relaxation. Increased renal perfusion pressure induces endothelial cells to release PGI₂ (71). On the other hand, increased medullary BP or flow induces the renal medullary interstitium and collecting ducts to release PGE₂. Young humans with a family history of HTN have defective vasodilator prostaglandin system and thereby increased renal vasoconstriction sensitivity to ANG II (124).

EDHF.

The endothelium may influence the renal vascular tone by releasing EDHF. EDHF hyperpolarizes endothelial cells by activating Ca²⁺-activated K⁺ channels. Hyperpolarization is transferred to VSM cells via myoendothelial coupling and gap junctions (connexins), or the efflux of K⁺ ions from hyperpolarized endothelial cells may accumulate in the extracellular space and act on the inward rectifier potassium channel and Na⁺-K⁺ pump to cause VSM hyperpolarization. The nature and mechanism of action of EDHF vary in different tissues and species. EDHF include K⁺ ion, epoxyeicosatrienoic acids (EETs), H₂O₂, and C-type natriuretic peptide (38). EDHF significantly affects RBF in human interlobar arteries (14). Also, in the rat renal microcirculation, even during inhibition of NOS and COX, there is residual vasodilatory response to acetylcholine, indicating the presence of EDHF (30). Administration of 40Gap 27, an inhibitory peptide homologous to the second extracellular loop of connexin-40, abolishes the residual acetylcholine vasodilatation, supporting a role for gap junctions in EDHF-mediated renal vasodilation. Also, administration of the 40Gap 27 inhibitory peptide, even in the absence of l-NAME and indomethacin, is associated with decreased RBF and increased BP, suggesting that tonic EDHF may maintain basal renal vascular tone and RBF (30). Cx40-deficient mice are hypertensive, suggesting that impaired EDHF-mediated renal vasodilatation may play a role in HTN (31). Also, while EDHF-mediated renal vascular relaxation is normal in young SHR, it is reduced in aged SHR and associated with depolarized resting membrane potential of the renal artery VSM (15), suggesting that decreased EDHF may be involved in the progression of HTN.

EETs and hydroxyeicosatetraenoic acids.

Epoxy and 20-hydroxy derivatives of arachidonic acid oxidation by cytochrome P-450 monooxygenase may also play a role in the regulation of renal vascular function, BP, and the pathogenesis of HTN. EETs are produced in the endothelium, proximal tubules, and collecting ducts, and some of the EETs are potent vasodilators and natriuretic compounds. Increasing the renal levels of EETs by inhibiting soluble epoxide-hydrolase reduces BP in SHR (193). Also, high-salt diet induces the expression of renal epoxygenases in Dahl salt resistant, but not hypertensive Dahl-SS rats. Furthermore, in mice, CYP4A10 deletion causes a decrease in EET urinary excretion and promotes SS-HTN (64). These studies have raised interest in cytochrome P-450-derived EETs as EDHFs in the kidney and as potential targets for antihypertensive therapies (64). It is important to note that the vascular effects of EETs may vary. For example, 11,12-EET is a stereoselective renal vasodilator, and 11,12(R,S)-EET, but not 11,12(S,R)-EET, increases the diameters of the interlobular and afferent arterioles. Also, the response to 11,12-EET is not affected by removal of the endothelium or by inhibition of COX with indomethacin. In contrast, 5,6-EET elicits vasoconstriction in the interlobular and afferent arterioles, and the vasoconstrictor response is abolished by removal of the endothelium or COX inhibition. These findings indicate that the preglomerular vasodilation to 11,12-EET is stereoselective and is the result of direct action of the epoxide on the preglomerular VSM, whereas the vasoconstriction to 5,6-EET is COX dependent and requires an intact endothelium (65).

20-Hydroxyeicosatetraenoic acid (HETE) may play a role in renal myogenic autoregulation, TGF control of afferent arterioles, and vasoconstrictor actions of ANG II and ET-1 (3, 104). In addition to its potent renal vasoconstrictor effects, 20-HETE is also natriuretic and, therefore, has counteracting effects in the control of BP. The natriuretic action involves inhibition of Na⁺-K⁺-ATPase in the proximal tubules and Na⁺-K⁺-2Cl⁻ cotransporter in thick ascending limb (60). The vasoconstrictor action involves blockade of Ca²⁺-activated K⁺ channels in VSM. Renal expression of CYP4A2, which produces 20-HETE, is increased in SHR, and CYP4A2 inhibitors attenuate HTN in SHR and DOCA SS-HTN (150). Also, in mice, deletion of CYP4A14 increases expression of CYP4A2, resulting in increased 20-HETE and HTN (64).

Hydroxy derivatives of arachidonic acid 12- and 15-HETE are produced by 12/15-lipooxygenase in the glomeruli, mesangial cells, and renal microvessel endothelial and VSM cells (195). 12- and 15-HETE constrict renal vessels and glomerular mesangial cells. 12-HETE induces VSM growth and partly mediates ANG II-induced afferent arteriolar vasoconstriction, as well as TGF-β and ANG II-induced mesangial cell hypertrophy and ECM accumulation (192). 12/15-Lipooxygenase inhibition or gene deletion blunts the pressor response to ANG II and blocks ANG II-induced hypertrophy and ECM accumulation in rat mesangial cells (85). Also, 12/15-lipooxygenase deficiency is associated with increased eNOS expression/activity, suggesting a relation between the two pathways in the control of vascular function (5). Urinary excretion of 12-HETE is increased in essential HTN. Also, 12-HETE production is increased in SHR, and 12-lipooxygenase inhibitors decrease BP in these rats (140).

Kinins and kallikriens.

Kinins (bradykinin, lysyl-bradykinin) have been implicated in the regulation of RBF, renal function, and BP (132). The vasodilator, diuretic, and natriuretic effects of kinins may be mediated by the release of NO, PGI₂, and EDHF. RAS tonically stimulates renal kinin production via a non-AT₁R pathway, possibly involving endothelial AT₂R (156). Kallikrein mRNA and protein are also expressed in vascular tissues. Renal kallikrein excretion is increased in humans and rats with mineralocorticoid/salt-dependent HTN, but is decreased in essential HTN (18).

ET.

Renal endothelial dysfunction may involve an increase in vasoconstrictive mediators, such as ET-1. ET is a potent vasoconstrictor peptide produced by the renal endothelial, mesangial, and tubular epithelial cells (88). Compared with other organs, the kidney expresses low levels of ET isopeptides (43). ET-1 is produced in the afferent and efferent arterioles, while ET-3 is produced in the proximal convoluted tubule, part of the distal tubule, and the cortical and outer medullary collecting duct. The inner medullary collecting duct produces both ET-1 and ET-3 (175). ET production is triggered by physical stimuli, such as shear stress, hypoxia, and osmolarity, by hormones such as insulin, adrenaline, arginine-vasopressin (AVP), and ANG II, and by peptides such as cytotoxin, thrombin, endotoxin, and TGF-β (107). However, the plasma level of ET is very low, and ET acts mainly locally in an autocrine or paracrine fashion.

ET acts via two main receptor subtypes, ET_AR and ET_BR. Both ET_AR and ET_BR are expressed in the afferent and efferent arterioles and the glomerulus (Fig. 6). Activation of ET_AR and ET_B2R in VSM induces vasoconstriction, while activation of ET_B1R in endothelial cells induces the release of NO and PGI₂ and vasodilation. ET_AR mediates vasoconstrictor actions of ET-1 in both afferent and efferent arteriole. ET_B2R mediates vasoconstriction in afferent arteriole, whereas ET_B1R mediates vasodilatation in efferent arteriole (67). ET_AR is more predominant in renal VSM, proximal convoluted tubule, and renal medullary interstitial cells. ET_BR are present in proximal straight tubule, collecting duct, and upper thick ascending limb (167). ET_AR recognizes ET-1 and ET-2, whereas ET_BR recognizes ET-1, -2, and -3 (62).

Fig. 6.

Differential distribution of receptors and mediators in the renal vasculature and nephron. Renal vascular receptors (shown in red) are activated by VCs to regulate RBF in afferent and efferent arterioles and vasa recta. VDs prevent excessive renal vasoconstriction. Decreased VDs and increased VCs lead to renal vasoconstriction and HTN. Receptors in tubules, loop of Henle, and collecting ducts (shown in brown) are modulated by substances that promote sodium retention or excretion and thereby maintain water and electrolyte balance, plasma volume, and BP. Increased sodium and water retention lead to increased plasma volume and HTN. AVP, arginine-vasopressin.

At low doses, ET-1 and ET-3 may activate mainly ET_BR and stimulate the release of NO and PGI₂ and thereby increase RBF and sodium and water excretion. In contrast, high doses of ET-1 and ET-3 activate mainly ET_AR and promote renal vasoconstriction and sodium and water absorption (83, 162). ET-1 induces vasoconstriction of afferent and, to a lesser extent, efferent arterioles, increases RVR, and decreases RBF (83). ET-1 also causes contraction of mesangial cells and decreases glomerular filtration surface area and GFR (6). ET may also mediate some of the renal actions of ANG II (130). ET-1 via ET_AR and ET_BR may also regulate eNOS and nNOS isoforms in the renal inner medulla (164).

ET-1 production is increased in the endothelium and the kidney of rat models of SS-HTN. ET-1 increases oxidative stress, vascular inflammatory response, and vascular remodeling. In rats, CYP450 hydroxylase and COX arachidonic acid metabolites may contribute to ET-1-induced afferent arteriolar vasoconstriction and VSM [Ca²⁺]_i (66). Also, chronic ET-1 infusion in rats is associated with vasoconstriction of afferent and efferent arterioles and increased urinary excretion of the oxidative stress biomarkers 8-iso-PGF_2α and thiobarbituric acid reactive substances, and these effects are attenuated by the O₂^•− dismutase mimetic tempol. ET stimulates O₂^•− formation in rat VSM, possibly via PKC-mediated increase in NADPH oxidase activity. The increased ET production in SS-HTN is also associated with increased oxidative stress (151). Also, chronic ET-1 infusion in rats on a high-salt diet is associated with enhanced vascular reactivity and Ca²⁺ influx into VSM (160). Interestingly, the renal vascular reactivity to ET-1 is greater in SHR than Wistar-Kyoto rats (173), but is blunted in DOCA SS hypertensive rats (143). Also, while the plasma levels of ET-1 may be low in SHR, neutralization of ET-1 by specific antibodies or ET_AR blockade decreases BP and RVR, increases renal plasma flow and GFR, and improves the renal hemodynamics and sodium excretion (82, 122, 173). ET_AR antagonists also reduce BP and vascular hypertrophic remodeling in ET-dependent rat models of SS-HTN (143).

ANG II.

ANG II is a major mediator of renal vascular dysfunction and vasoconstriction. ANG I and ANG II levels are higher in the kidney than the plasma, suggesting that the intrarenal ANG II levels are not due to simple equilibration with circulating ANG II. Studies on rat have demonstrated that the concentrations of total immunoreactive angiotensin (reflecting ANG II) in glomerular filtrate and early, mid-, and late proximal tubule fluid averaged 29–40 nM, compared with 32 pM in systemic plasma. Also, angiotensin concentrations in the plasma of star vessel (end of efferent arteriole) exceeded systemic levels by a factor of 1,000. These observations have suggested that substantial amounts of angiotensin peptides are released into or generated within intrarenal fluid compartments, that the components necessary for ANG II formation are present in these sites, and that local ANG II is likely to affect the regulation of renal function independently of systemic ANG II (118, 152).

ANG II stimulates both AT₁R and AT₂R. AT₁R is more widely expressed in the kidney than AT₂R, and AT₁R density is greater in the medulla than the cortex. The high ANG II levels and AT₁R density in the medulla support a major role in regulating hemodynamic and tubular function. AT₁R are also localized on VSM cells of the renal vasculature, including afferent and efferent arterioles (Fig. 6). ANG II contributes to regulation of salt and water balance directly by activating the NHE3 transporter and stimulating sodium reabsorption in the proximal tubules and indirectly by stimulating Aldo secretion in the adrenals (97, 182). Also, renin secretion in JGCs is controlled by negative feedback loop involving ANG II and intrarenal pressure. Communication between endothelial cells and renin secreting JGCs through connexin 40-dependent gap junctions may mediate the Ca²⁺-dependent inhibitory effects of ANG II and intrarenal pressure on renin synthesis (181).

Some of the components of the RAS are upregulated in certain forms of HTN. The intrarenal angiotensinogen mRNA is increased in Dahl SS rats on a high-salt diet (87). Increased systemic and renal vascular sensitivity to ANG II is observed in normotensive men with a family history of HTN (187), and in SHR even in the developmental phase of HTN (21, 180). Also, sympathetic nerves enhance the renovascular responses to ANG II in SHR (34). The ability of PGI₂ to attenuate ANG II-induced vasoconstriction is also reduced in the kidney of SHR (72). Interestingly, low-dose infusion of ANG II for 6–10 days in rabbits, rats, or mice causes increases in RVR, BP, oxidative stress, urinary 8-iso-PGF_2α, ONOO⁻ generation, and nitrotyrosine deposition. ANG II induces ET-1 production, which increases O₂^•− and decreases NO bioavailability. ANG II also promotes the renal production of TxA₂, which, in turn, causes renal vasoconstriction (177). The effects of ANG II on multiple systems and vascular mediators may explain the elevated BP in essential and renovascular HTN where the ANG II levels may be normal (133).

The net effects of ANG II on the vascular and renal mechanisms and BP may also depend on the duration of increased ANG II activity. Short-term infusion of ANG II in rats causes an elevation in BP that returns to normal on cessation of ANG II. In contrast, prolonged ANG II infusion for 6–10 days causes long-lasting elevation in BP, afferent and efferent arteriolar constriction, and reduced GFR that persists after cessation of ANG II infusion (46). Long-term ANG II may cause injury to the renal microvessels and peritubular capillary and tubulointerstitium, leading to structural changes, such as afferent arteriolar wall thickening and loss of peritubular capillaries, as well as functional changes, such as decreased NO and increased ET-1. The ANG II-induced tubulointerstitial injury also promotes the release of proinflammatory chemokines and leukocyte adhesion proteins and infiltration of mononuclear cells, which express ANG II at the site of injury and thereby further augment intrarenal ANG II (77). The increased ANG II leads to further renal vasoconstriction, microvascular wall thickening, decreased glomerular filtration, sodium retention, and SS-HTN (45).

TxA₂ and PGE₂.

TxA₂ is another prostanoid produced by COX and an important mediator of the renal hemodynamic and pressor effects of ANG II. TxA₂ is a potent vasoconstrictor in the renal vessels and afferent arterioles, acting via thromboxane-prostanoid (TP) receptors to increase renal VSM [Ca²⁺]_i (Fig. 7). TxA₂ may also cause eNOS activation and NO production in endothelial cells by increasing [Ca²⁺]_i due to activation of endothelial TP receptor or through gap junctions from VSM (147). In isolated, microperfused rabbit afferent arterioles, TP receptor activation produces ROS, which promotes vasoconstriction, but also produces NO, which counteracts vasoconstriction (147). An increase in endogenous levels of TxA₂ may contribute to the enhanced TGF activity in young SHR, leading to a rightward shift in the pressure-natriuresis curve and HTN (11). TP receptors may also mediate the effects of the F2 isoprostane 8-iso-PGF_2α on preglomerular vasoconstriction. The renal vascular reactivity to TxA₂ is increased in SHR, possibly due to increased affinity/number of renal TP receptors (21).

Fig. 7.

Mechanisms of renal VSM contraction in HTN. Increased VC agonists, such as ET-1, ANG II, or phenylephrine (Phe), activate their receptor (R), stimulate PLC-β, and increase IP₃ and DAG. IP₃ stimulates Ca²⁺ release from the SR. Ca²⁺ stimulates ryanodine-sensitive receptors (RyR) in SR and further releases intracellular Ca²⁺ [Ca²⁺-induced Ca²⁺ release (CICR)]. Agonists also stimulate Ca²⁺ influx through voltage-gated (VGC), ligand-gated (LGC), and store-operated channels (SOC). Ca²⁺ binds calmodulin (CAM), activates myosin light chain (MLC) kinase (MLCK), causes MLC phosphorylation, and initiates renal VSM contraction. Ca²⁺ mobilization mechanisms are normally counterbalanced by Ca²⁺ and Na⁺ extrusion via plasmalemmal Ca²⁺-ATPase, Na⁺-K⁺-ATPase, and NCX and NHE to maintain [Ca²⁺]_i and intracellular pH. DAG activates PKC. PKC induces phosphorylation (P) of CPI-17, which inhibits MLC phosphatase and enhances myofilament Ca²⁺ sensitivity. PKC-induced phosphorylation of calponin (Cap) allows more actin to bind myosin. PKC also activates protein kinase cascades involving Raf, MAPK kinase (MEK), and MAPK, leading to phosphorylation of the actin-binding protein caldesmon (CaD). Also, RhoA/Rho kinase inhibits MLC phosphatase and further enhances the Ca²⁺ sensitivity of the contractile proteins. Significant increases in renal VSM [Ca²⁺]_i and the Ca²⁺-sensitization pathways of VSM contraction cause renal vasoconstriction, increased RVR, and HTN. AA, arachidonic acid; G, GTP-binding protein; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PS, phosphatidylserine.

PGE₂ elicits both vasodilatory and vasoconstrictor actions in afferent arterioles. In rats, vasodilation is mediated by EP₄ receptors and G_αs protein, increased cAMP, and decreased VSM [Ca²⁺]_i (126). The vasoconstriction of afferent arterioles and interlobular arteries is mediated by EP₃ receptors and G_αi (166, 178). In mice, EP₂ receptors partly mediate PGE₂-induced afferent arteriolar vasodilation and buffer the vasoconstriction mediated by EP₁ and EP₃ receptors and ET-1 (63). ANG II via AT₁R also tonically stimulates renal PGE₂ production (156). Spontaneous increase in tone above initial tension is observed in renal arteries of SHR, likely due to reduced NO production and large-conductance, calcium- and voltage-activated potassium channel activity, leaving the actions of endogenous COX products that activate TxA₂/TP and PGE₂/EP₁ receptors unopposed (111).

ATP and adenosine.

ATP and adenosine play a role in the regulation of renal function through their action on the renal microcirculation and tubular function (69). Purinoceptors P1 and P2 are widely distributed in the kidney (52, 68) (Fig. 6). P1 receptor has more affinity to adenosine, whereas P2 receptor is more toward ATP. ATP released from the macula densa in response to increased NaCl delivery is broken down to AMP in the extracellular space, then acted upon by ecto-5′-nucleotidases to form adenosine, a mediator of TGF (89, 129) (Fig. 3). O₂^•− may also increase the production and renal vasoconstrictor actions of adenosine (23).

Both P2X and P2Y receptors increase VSM [Ca²⁺]_i; P2X induces Ca²⁺ influx, while P2Y stimulates intracellular Ca²⁺ mobilization (42). The renal vascular responsiveness to ATP is increased in SHR, possibly due to nonspecific functional changes in the renal vasculature, rather than specific alterations in the renal purinoceptors (42).

It is important to note that the vasoconstrictor stimuli vary along the renal arterial arcade. ANG II mainly constricts small resistance renal arterioles, while TxA₂ and AVP act on the interlobar and arcuate arteries, causing variable degrees of renal vasoconstriction. Adenosine affects TGF, while ATP acts on P2X receptors in the renal vascular, glomerular, mesangial, and tubular epithelial cells to regulate the renal microvascular function, RBF, and autoregulation, as well as the renal hemodynamics and tubular function (68, 69, 171, 186).

Also, although the medullary microcirculation receives only ∼10% of RBF, it plays a critical role in long-term regulation of BP. Pressure diuretic-natriuretic responses are associated with changes in the medullary blood flow and the renal interstitial hydrostatic pressure. Also, pericytes are contractile smooth muscle-like cells that surround descending vasa recta and thereby affect medullary blood flow. Known vasoconstrictors of descending vasa recta include ANG II, ET-1, norepinephrine, TxA₂, AVP, and adenosine. Vasodilators include NO, PGE₂, acetylcholine, and bradykinin (Fig. 6). The vasa recta blood flow is subjected to tonic vasoconstrictor influence of ANG II and vasodilator influence of prostaglandins (26). NO produced from eNOS regulates baseline blood flow in the mouse renal medulla, and NO produced by nNOS is an important buffer of the constrictor actions of ANG II (109). The specific role of NO in the medulla is supported by the observation that acute and chronic NOS inhibition in the renal medulla decreases medullary blood flow and reduces sodium and water excretion in the absence of changes in cortical blood flow (108).

Renal VSM Dysfunction in HTN

Renal VSM dysfunction involves an increase in basal tone and VSM reactivity to vasoconstrictors and increased signaling pathways of VSM contraction. In VSM, agonists, such as ANG II, ET-1, and norepinephrine, act via G protein-coupled receptors to activate phospholipase C, which stimulates the hydrolysis of phosphatidyinositol bisphosphate into inositol trisphosphate (IP₃) and diacyglycerol (DAG) (Fig. 7). IP₃ stimulates Ca²⁺ release from the sarcoplasmic reticulum, and DAG activates PKC. Vasoconstrictor agonists also increase Ca²⁺ influx from the extracellular space (116). Ca²⁺ binds calmodulin to form a complex that activates myosin light chain kinase, causes myosin phosphorylation, and initiates actin-myosin interaction and VSM contraction. PKC, Rho-kinase, and mitogen-activated protein kinase (MAPK) contribute to VSM contraction by increasing the [Ca²⁺]_i sensitivity of contractile proteins (59, 138).

[Ca²⁺]_i.

VSM [Ca²⁺]_i is a major determinant of the myogenic mechanism of renal vascular autoregulation and agonist-induced vasoconstriction (116, 131). The renal microcirculation has heterogeneous distribution of Ca²⁺ channels and differential Ca²⁺ influx in various vascular segments. The renal cortical preglomerular afferent arteriole, juxtamedullary efferent arteriole, and outer medullary vasa recta have L- and T-type voltage-gated Ca²⁺ channels (VGCCs), whereas the mid-to-outer cortical efferent arteriole does not (57). ANG II-induced vasoconstriction of afferent arteriole involves depolarization and VGCCs, whereas, in efferent arteriole, receptor-operated and store-operated Ca²⁺ channels are involved (102). The vasoconstriction mechanisms in the renal medulla are similar to the preglomerular vasculature and involve depolarization and VGCCs (57).

In the afferent arteriole, ANG II, ET-1, or norepinephrine causes an increase in [Ca²⁺]_i that is augmented by activation of ryanodine receptor and Ca²⁺-induced Ca²⁺ release in the sarcoplasmic reticulum. Cyclic adenosine diphosphate ribose (ADPR), produced from β-nicotinamide adenine dinucleotide by membrane-bound ADPR cyclase, sensitizes ryanodine receptor to further amplify Ca²⁺-induced Ca²⁺ release and augment [Ca²⁺]_i and renal vasoconstriction (96, 168). ET-1 increases [Ca²⁺]_i by stimulating the release of intracellular Ca²⁺ and Ca²⁺ influx from the extracellular medium (149). In the afferent arteriole, ET-1 also increases the formation of O₂^•−, which activates ADPR cyclase and accounts for 60% of subsequent Ca²⁺ signaling (39). Also, in the afferent arteriole, epidermal growth factor receptor (EGFR) tyrosine kinase may contribute to ANG II-induced [Ca²⁺]_i by promoting Ca²⁺ influx (22).

Renal vascular reactivity to AVP and ANG II is increased in SHR (56). AVP-induced vascular reactivity and RVR are attributed to increased density of V1 receptor in the renal interlobular arteries and augmented VSM [Ca²⁺]_i due to Ca²⁺ mobilization from the intracellular stores and Ca²⁺ influx through plasma membrane (37). In SHR, the enhanced vascular reactivity to ANG II is due to increased number/affinity of AT₁R and may also involve increased basal [Ca²⁺]_i, PKC activity and sensitivity of myosin light chain to Ca²⁺, and reduced cAMP generation due to ANG II-induced increase in formation of vasoconstrictive prostaglandins from arachidonic acid (134, 142, 180). Ca²⁺ signaling and store-operated Ca²⁺ entry are exaggerated in preglomerular VSM of young SHR (40). Also, diminished NCX and Ca²⁺ sequestering/extrusion mechanisms via sarcoplasmic reticulum and plasmalemmal Ca²⁺-ATPase could contribute to increased [Ca²⁺]_i in afferent arteriolar VSM of SHR (8, 120).

PKC.

Phospholipase C isoforms-β, -γ, and -δ are upregulated in systemic and renal vessels of SHR, supporting their role in enhancing vascular reactivity in genetic HTN (125). Activation of phospholipase C increases DAG production, which activates PKC. Classical α-, βI-, βII-, and γ-PKC isoforms are activated by DAG, phorbol esters, and Ca²⁺. Novel δ-, ɛ-, η-, θ-, and μ-PKCs are activated by DAG/phorbol esters, but are Ca²⁺ independent. Atypical ζ- and λ/ι-PKCs are independent of DAG or phorbol esters, but activated by other lipid mediators (138). The role of Ca²⁺ release, Ca²⁺ influx, and PKC varies between agonists, with ANG II depends on the three factors, TxA₂ independent of Ca²⁺ and AVP has intermediate dependence (19).

PKC plays a role in the regulation of renal vascular function and tubular transport systems. In rabbit, both the afferent and efferent arterioles express NCX, which is activated by PKC (44, 113). PKC is also involved in the regulation of Na⁺-K⁺-ATPase by ANG II, dopamine, ET, and norepinephrine. PKC-mediated activation of Na⁺-K⁺-ATPase in basolateral membrane of the renal tubular cells drives sodium reabsorption throughout the nephron and thereby affects extracellular fluid volume and BP (41).

Studies in systemic vessels have shown that α-PKC enhances Ca²⁺-dependent VSM contraction and is overexpressed in HTN (138). Ca²⁺-independent ɛ-PKC may increase the myofilament sensitivity to [Ca²⁺]_i in VSM. δ-PKC is mainly associated with the cytoskeleton and may play a role in vascular remodeling. ζ-PKC is localized in the nucleus and may promote VSM growth and vascular hypertrophic changes associated with HTN (138). Studies in cannulated rat femoral arterial branches have shown that high pressure induces O₂^•− production via PKC-dependent activation of NADPH oxidase (176). It has also been shown that the increased NADPH oxidase-mediated O₂^•− production in renovascular HTN involves PKC (58). Further studies are needed to investigate whether increased PKC expression/activity in VSM enhances renal vasoconstriction and trophic vascular changes and leads to increased RVR and HTN.

RhoA/Rho kinase.

RhoA/Rho kinase signaling plays a role in maintaining VSM contraction in response to stretch and receptor-mediated activation. NO causes vasodilation partly through inhibition of Rho kinase, and maintenance of renal basal vascular tone depends on a balance between NO production and Rho/Rho kinase signaling. This is supported by the observation that RhoA/Rho kinase signaling is amplified in eNOS knockout mouse (141).

Rho kinase is involved in the renal basal vascular tone, the myogenic contraction of preglomerular and afferent arterioles, and the vasoconstriction to ANG II, TxA₂, and AVP (117). Also, the effect of ROS on vascular tone and VSM contraction may be mediated by Rho/Rho kinase. Increased NADPH-dependent ROS and reduced NO bioavailability activate Rho kinase, which, in turn, inhibits myosin light chain phosphatase, leading to augmented myosin light chain phosphorylation and VSM contraction (75).

While the expression of Rho guanine nucleotide exchange factor, a positive regulator of Rho kinase, is similar in Wistar-Kyoto and young 4-wk-old SHR, it is increased in SHR with established HTN, suggesting that increased Rho/Rho kinase signaling is likely a consequence of HTN (191). On the other hand, RhoA/Rho kinase may participate in the pathogenesis of nephrosclerosis in SHR, partly by upregulation of gene expressions of oxidative stress, ECM, adhesion molecules, and antifibrinolysis (121).

MAPK.

MAPK is one of several kinases that plays a role in the transduction of extracellular mitogenic signals to the nucleus. MAPK is a Ser/Thr protein kinase that is fully activated by dual phosphorylation at Thr and Tyr residues and in turn activates nuclear transcription factors and VSM growth and proliferation. In growth-arrested human VSM cells, MAPK is mainly cytosolic, but translocates into the nucleus during activation by mitogens (112). In cultured VSM cells, ANG II-induced activation of MAPK and mitogenic effects requires Ca²⁺-dependent transactivation of EGFR, because it provides docking sites for the upstream tyrosine kinase c-Src and the downstream adaptors Shc and Grb2 at the plasma membrane, which are essential for MAPK activation (36). MAPK, in turn, phosphorylates and activates several kinases and transcription factors and induces the nuclear protooncogene c-fos (25). Tyrosine phosphorylation events involving EGFR tyrosine kinase and rapid c-Src activation also contribute to the renal microvascular and afferent arteriolar vasoconstriction and increased [Ca²⁺]_i and Ca²⁺ influx in response to ANG II (22). ET-1-induced activation of PKC and MAPK also plays a role in VSM contraction and growth (17). Also, in differentiated VSM cells, MAPK causes phosphorylation of the actin-binding protein caldesmon, thus allowing more actin to interact with myosin, and thereby enhances VSM contraction (138) (Fig. 7).

MAPK-mediated VSM growth may underlie the vascular hypertrophic changes in HTN. MAPK activity is enhanced in the vasculature of SHR (92). Also, the renal VSM expression/activity of EGFR is increased in Dahl-SS rats before the onset and in established HTN. EGFR mediates the fibrogenic and vasoconstricting actions of ANG II and ET-1, and inhibition of EGFR attenuates the HTN caused by ANG II and the renal vascular fibrosis in l-NAME-induced HTN (47, 190).

ECM and Renal Vascular Remodeling in HTN

The renal vasculature undergoes significant remodeling, fibrotic changes, and structural alterations in HTN (157). Renal vascular remodeling is associated with VSM growth and proliferation and increased deposition of ECM proteins, especially collagen types I, III, and IV in resistance arteries, glomerulus, and interstitium. NO inhibits collagen I gene activation, and chronic NOS inhibition is associated with activated collagen I gene and accumulation of ECM in the renal vasculature. Also, ANG II causes collagen gene I activation and fibrogenic action in the renal vasculature, independent of hemodynamics, and likely mediated by ET-1, MAPK, and TGF-β (169). The fibrogenic action of ET-1 involves EGFR-mediated activation of MAPK p42/44 and collagen I gene (47). ANG II-induced ET-1 expression in adventitial fibroblasts is partly mediated by NADPH oxidase, and ET-1 in turn stimulates collagen formation, implicating oxidative stress in vascular remodeling (4).

During vascular remodeling, matrix metalloproteinases (MMPs) degrade ECM proteins and adhesion molecules, thus enabling VSM cells to migrate and proliferate, and inflammatory cells to infiltrate the vessel wall (179). Endogenous tissue inhibitors of MMPs provide a balancing mechanism and prevent excessive ECM degradation (99). Several MMPs are released in the vasculature in response to inflammatory cytokines and transcription factors and contribute to vascular remodeling by promoting ECM degradation. Additional effects of MMPs on the endothelium and VSM have also been described (127). The role of MMPs in the regulation of the renal vasculature and kidney function needs to be thoroughly examined.

In SHR, all renal arteries except preglomerular arterioles exhibit increased media thickness and VSM cell number and volume. Prehypertensive SHR also exhibit increased cross-sectional arterial media (159). In the vasculature of adult SHR, a decrease in MMP-1, -2, and -3 may contribute to remodeling of resistance arteries and the setting of HTN (70). Also, the expression of integrins is abnormal in blood vessels of SHR. Integrins act as physical “joints” between ECM and cytoskeletal components and as signal-transducing receptors. Changes in the adhesion receptor integrins lead to abnormal interaction between VSM cells and ECM proteins and changes in the vessel structure. Also, the interaction of arginine-glycine-aspartate (RGD) containing peptides and fibronectin with α₅β₁-integrin promotes VSM growth and is enhanced in SHR vessels. Osteopontin, a glycoprotein that interacts with integrin and increases VSM cell proliferation, is also overexpressed in HTN (127).

Perspectives and Significance

Research into the renal vascular mechanisms of HTN demonstrates complex interaction between multiple genetic, environmental, and dietary factors and numerous cellular mediators of renal vascular dysfunction. Gene mutations/polymorphism in ion channel, transporter, or enzyme, combined with an adverse environmental or dietary factor, could lead to renal vascular dysfunction, renal tissue injury, and HTN. Multiple gene mutations are associated with HTN, and advanced genetic engineering should help to delineate more specific gene targets. Gene therapy designed to target the specific gene defect may provide new antihypertensive treatment. Successful management of HTN may also require rigorous control of environmental risk factors, particularly dietary salt intake and the associated increases in oxidative stress and vascular inflammation. Other factors related to age, ethnic background, sex, body weight, and comorbidities, such as diabetes, should also be considered in the design of antihypertensive therapy. Reduction of oxidative stress using antioxidants and management of vascular inflammation using cytokine antagonists may be useful. ACE inhibitors remain a major antihypertensive therapy to reduce ANG II production, salt and water retention, and ANG II-dependent renal vasoconstriction and HTN. ANG II, ET-1, TxA₂ receptor blockers, Ca²⁺ channel blockers, and inhibitors of PKC, Rho kinase, or MAPK could reduce VSM mechanisms of renal vasoconstriction. Modulators of MMPs and tissue inhibitors of MMP activity could reduce ECM accumulation and renal vascular remodeling. A combination of these treatment strategies should be most effective in HTN.

GRANTS

This work was supported by grants from the National Heart, Lung, and Blood Institute (HL-65998 and HL-70659).